1. Field of the Invention
This application relates in general to a locking mechanism for a pneumatic differential engine for power-operated doors and, more particularly, to a mechanical locking device for maintaining the differential engine in the “door closed” position, as well as an emergency door-opening device enabling manual opening of the doors.
2. Description of Related Art
Pneumatic cylinders have been utilized in mechanical systems to convert compressed air into linear reciprocating movement for opening and closing doors of passenger transportation vehicles. An example of this type of door actuating system is shown in U.S. Pat. No. 3,979,790.
Typically, pneumatic cylinders used in this environment consist of a cylindrical chamber, a piston and two end caps hermetically connected to the cylindrical chamber. The end caps have holes extending therethrough to allow the compressed air to flow into and out of the cylindrical chamber, to cause the piston to move in a linear direction, and to apply either an opening or closing force to the vehicle door.
Pneumatic cylinder/differential engine systems have also been designed for opening and closing doors of passenger transportation vehicles. Examples of these systems are shown in U.S. Pat. Nos. 4,231,192; 4,134,231; and 1,557,684. None of these currently used systems have a locking system for locking the doors in a closed position should the system experience a loss of air supply pressure.
To understand the locking mechanism of the present invention, it may be helpful to understand how a pneumatically powered differential engine door opening device operates.
Reference is now made to FIG. 1, which schematically shows a pneumatically-powered differential engine door opening device. The differential engine includes a housing comprising a large diameter cylinder 1 and a small diameter cylinder 2, closed at their ends by large cap 6 and small cap 7. A large diameter piston 4 is installed in the large cylinder 1 and a small diameter piston 5 is installed in the small cylinder 2. A toothed rack 16 is attached to and extends between the large piston 4 and small piston 5. The toothed rack 16 is engaged with a pinion gear 15. The pinion gear 15 is, in turn, connected to a shaft 14 which drives the mechanism for closing and opening the vehicle door. Linear movement of pistons 4 and 5 causes linear movement of the toothed rack 16. This linear movement is converted into rotational movement of the pinion gear 15 and shaft 14 causing opening and/or closing of the vehicle door. As viewed in FIG. 1, movement of the pistons 4 and 5 toward large cap 6 or to the left, causes an opening of the doors, and movement of pistons 4 and 5 toward small cap 7 or to the right, causes a closing of the doors.
As shown in FIG. 1, the outer side of the small cylinder 2 is connected through an opening 19 in the cap 7 to a reservoir of compressed air that constantly applies a positive pressure to the surface 5a of small piston 5 facing opening 19. As shown schematically in FIG. 1A, the large cap 6, attached to the outer end of the large cylinder 1, has a chamber 17 including holes 9 and 10 which are connected through a port 80 to a three-way valve, which provides connections to a source of compressed air and to an exhaust. During closing of the doors, hole 9 is connected to a source of pressurized air and exhaust hole 10 is closed. Because the surface area of piston 4 is greater than the surface area of piston 5, the pistons 4, 5 move toward small cap 7 or to the right as shown in FIG. 1, rotating the pinion gear 15/shaft 14 in a counter-clockwise direction. During an opening stroke, holes 9, 10 are connected to an exhaust, causing the air to flow out of large cylinder 1. Because the small piston 5 is constantly attached to a source of positive air pressure, the exhausting of the air pressure from within the large cylinder 1 causes the pistons 4, 5, connected by toothed rack 16, to move toward large cap 6 or toward the left as shown in FIG. 1, within the large and small cylinders 1, 2. This movement toward the large cap 6 rotates the pinion gear 15/shaft 14 in a clockwise direction to initiate opening of the doors.
It has been determined in some instances that there is a need to slow the movement of the piston at the end of the stroke when opening and/or closing the door. A known technique for slowing this stroke is by restricting the flow of the exhaust air out of the cylindrical chamber. This is commonly known as cushioning the movement of the piston.
In this design, cushioning at the end of the opening piston stroke occurs through the use of a small hole 11 having a diameter that is substantially smaller than that of opening 82. This hole 11 is located at a side surface of chamber 17, which provides connection to the inside volume of the chamber of the large cylinder 1. A cylindrical sealing disk 8 is installed between the piston 4 and cap 6 and is supported between two springs 12, 13. The movement of the pistons 4, 5 toward large cap 6 or to the left as shown in FIG. 1, causes compression of springs 12, 13 bringing the disk 8 into contact with a face 17a of chamber 17, forming a seal with the chamber face 17a. Once this seal is achieved, air can no longer exit the chamber of the large cylinder 1 through opening 82 into chamber 17 and, thus, can only exit through hole 11 into chamber 17. Since the diameter of hole 11 is smaller than the diameter of opening 82, the flow of the air out of the large cylinder 1 is restricted, consequently slowing down the speed of the opening piston stroke movement to the left and achieving a cushioning effect during opening of the doors.
U.S. Pat. No. 2,343,316 teaches a pneumatic cylinder/differential engine for power-operated doors, wherein cushioning occurs near the end of the piston stroke during closing of the doors in order to prevent slamming. In this device, cushioning occurs when a sealing disk contacts with the surface of a cap, causing the exhaust air to flow through a small hole which significantly reduces the rate of flow of the exhaust air from the cylinder housing and decreases the linear speed of the piston.
As stated above, currently used pneumatic differential engines for power-operated doors do not have a locking mechanism for locking the doors in a closed position. The capability of locking the differential engine in a “door closed” position would be highly desirable, as it would ensure that the doors remained closed even in the event of partial or complete loss of air-supply pressure.